This invention relates to normally tacky and pressure-sensitive adhesives (PSAs), especially to PSAs that adhere firmly to polyolefin surfaces. A particularly preferred embodiment of the invention is a pressure-sensitive adhesive tape capable of functioning as a closure for disposable diapers.
The term "pressure-sensitive' is commonly used to designate a distinct category of adhesive tapes and adhesives which in solvent-free form are aggressively and permanently tacky at room temperature; see Houwink and Salomon, Adhesion and Adhesives, Elsevier Publishing Co., Amsterdam, Netherlands, 1967, p. 387). Such adhesives are recognized to possess a "four-fold balance" of adhesion, cohesion, stretchiness, and elasticity.
The earliest PSAs used in the manufacture of PSA tapes were the so-called "rubber-resin" compositions, in which natural rubber was blended with wood rosin tackifier. Over the years during which PSAs have been in widespread use, natural rubber has frequently been replaced by synthetic rubbers, including the so-called three-block ABA block copolymers, in which A is typically styrene and B is typically isoprene or butadiene. U.S. Pat. No. 3,239,478 teaches that, when an ABA rubber is employed in rubber-resin adhesives, it is necessary to incorporate large amounts of extender oils that are substantially compatible with the B block to achieve satisfactory tack and flexibility. The addition of oils is especially important when the B block is polybutadiene, which is much harder to tackify than polyisoprene. This patent also states that when extender oils are included, a large variety of tackifying resins, including rosin esters, can be employed.
U.S. Pat. No. 3,787,531 suggests that high shear rubber-resin adhesives can be prepared from a blend of a two-block, or AB, rubber and a compatible tackifying resin. The patent indicates that minor amounts of ABA polymers can be added to the composition to increase shear and tensile strength "without severely affecting tack or quick-stick properties", but the experience of those skilled in the PSA art has taught them that in such event it is necessary to include substantial amounts of the extender oil referred to in U.S. Pat. No. 3,239,478. U.S. Pat. No. 4,102,835 describes rubber-resin adhesives in which the rubber is a blend of minor amounts of ABA polymer and major amounts of either a rubbery homopolymer or a random styrene butadiene rubber (SBR). U.S. Pat. No. 3,658,740 states that blends of SBR and linear block copolymer are incompatible and hence not usable as adhesives when tackified with rosin ester.
U.S. Pat. No. 4,379,806, while primarily devoted to pressure-sensitive adhesive tapes in which a rubber-resin adhesive layer and a backing are simultaneously extruded, suggests in somewhat sweeping terms that the rubber portion of the PSA can be based on AB rubber and/or ABA polymers, optionally including up to about 50% of highly broken down natural rubbers, butadiene-styrene random copolymer rubbers, synthetic polyisoprene, chloroprene rubbers, nitrile rubbers, butyl rubbers and the like. Eight of the nine working examples, however, describe adhesives that are based on styrene:isoprene:styrene ABA polymers, up to 75% of the total rubber optionally being styrene:isoprene AB polymer, the system being tackified with synthetic polyterpenes; the ninth example describes an ethylene:vinyl acetate copolymer rubber tackified with a rosin ester.
Special considerations are involved in developing a pressure-sensitive adhesive that will adhere well to a low energy polyolefin surface such as the polyethylene film cover or the porous nonwoven fibrous polyester or polypropylene liner of a disposable diaper. It has been discovered, for example, that natural rubber tackified with a rosin ester does not age as well as if it is tackified with poly-.beta.-pinene or hydrocarbon resin. A blend of rosin ester with either SBR or AB polymer lacks sufficient tack to be useful.